Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of electrical logging apparatus. In the first category, one or more measurement electrodes—current source(s) or sink(s)—are used in conjunction with a return electrode (which may be a diffuse electrode such as a logging tool's body or mandrel). A measurement current flows in a circuit that connects a current source to the measurement electrode(s), through the earth formation to the return electrode, and back to the current source in the tool. In a second category, that of inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The measured responses are affected by properties of the earth formation including electrical conductivity, magnetic permeability, dielectric permittivity and the pore volume or porosity of the rock matrix and water saturation.
The dielectric constant of the formation may be estimated by transmitting an electromagnetic (EM) wave into the formation, and receiving it at one or more receivers (e.g., at receiver antennas). Then, the attenuation and phase shift between the received signals and the transmitted signals are determined, which are used to estimate the dielectric constant of the formation. Alternatively, the attenuation and phase shift between spaced receivers may be used to estimate the dielectric constant of the formation.
Multi-frequency dielectric tools can detect formation texture and water content. By combining complex permittivity measurements with measurements from other borehole devices (e.g., total formation porosity), the water salinity of the formation and water filled porosity of the formation can be estimated.